1. Field of Invention
The present invention generally relates to allograft bone devices for surgical implantation into bone tissue and particularly to a composite allograft bone device constructed from two or more separate bone pieces made from allograft, autograft and xenograft bone tissue that can be constructed to have dimensions that are larger than the dimensions of naturally occurring bone suitable for implantation in a surgical site.
2. Description of the Prior Art
Allograft bone tissue is widely used in orthopedic, neuro-, maxillofacial, podiatric and dental surgery. Allograft tissue is valuable in these fields of surgery because it is strong and it biointegrates well over time with the recipient patient's tissue. Allograft bone tissue can be shaped for specific surgical applications by the surgeon or by a bone product manufacturer in a manufacturing environment before the allograft bone tissue is transferred to the surgeon. Unfortunately because of the size limitation of the bone material only devices of a certain size could be constructed.
Surgical implants constructed entirely from allograft bone tissue are generally superior to implants constructed from synthetic or nonabsorbable polymers or metals because allograft bone tissue is bioinert and integrates well with the surrounding tissues.
Allograft bone occurs in two basic forms: 1) cancellous bone (also referred to as trabecular bone) and 2) cortical bone. Cortical bone is highly dense and has a compound structure comprised of calcium hydroxyapatite reinforced with collagen fiber. This cortical bone material is the predominant load bearing component of long bones in the human body. Many shapes and forms can be fabricated from allograft cortical bone tissue including pins, screws, plates, intervertebral discs and the like for use in human surgery. Cortical bone has one serious limitation that plastics and metal do not have. Bone parts and bone products made from allograft cortical tissue are limited in size, dimension and shape because of the anatomical limits on the thickness and length of the source bone.
As an example, the largest long bone, the femur bone, has a thick cortical wall that varies in thickness from about two millimeters to about ten millimeters. The majority of the femoral cortical bone wall typically ranges from about three millimeter to about eight millimeters in thickness. The length of the cortical tissue is also naturally limited by the size and the weight of the allograft tissue donor. Accordingly, specific implants fabricated from cortical bone have previously not been larger than these natural anatomical dimensions. The other long bones of the bones of the human body, the humorous, the tibia, the fibula, the radius, the ulna, the ribs, etc., are similarly limited in dimension. Shaped implants made from these other long bones are also necessarily limited in dimension.
The dimensional limit that has been achieved with single piece cortical bone is about 10 mm.times.about 13 mm cross-sections. The length of these sections can be much longer as they are taken from the long axis of the bone. The research that has been completed shows femoral sections ranging from 3 mm.times.4 mm to 10 mm.times.10 mm at the mid-shaft and tibial sections 3 mm.times.6 mm to 10 mm.times.13 mm at the proximal end.
Many medical problems and surgical procedures require implants larger than have previously been made out of allograft cortical tissue. It is desirable to have a surgical implant made entirely out of allograft cortical tissue that is larger than can be made from a single piece of naturally occurring bone. Two requirements must be met by any implant fabricated entirely from cortical bone, however, to achieve a successful surgical result. First, the components must be held together in such a way that the mechanical structure of the implant is not compromised during the surgical implantation procedure. Many surgical implants are implanted in the recipient patient with a large applied force. Many implants are actually hammered in as is the case for an intervertebral implant. Second, the compound structure of the implant must hold together during the post-operative period during which the allograft tissue is resorbed and remodeled.
The prior art contains many references directed to fasteners, spinal cages and devices which are constructed of inert metals or plastics which are used in bone repair. There are relatively few devices used in bone repair constructed of allograft bone because of the difficulty in obtaining and shaping the material and the natural limits placed on the size of the device based on the constraints of the sizes of the natural bone which can be shaped to form devices of a larger required size.
One example of an allograft device is disclosed in U.S. Pat. No. 4,877,020 which shows a dowel made of bone having a helicoidal thread.
Another device is shown in U.S. Pat. No. 4,932,973 where the use of a perforated bone matrix for use in insertion or implantation in a bone mass to promote bone growth is disclosed.
Similarly U.S. Pat. No. 5,112,354 discloses the preparation of an allograft bone segment for use in skeletal reconstruction. The bone segment is demineralized and a multiplicity of bores (described as pores) are drilled into the bone mass in a pattern to maximize the surface area of the implant. Some of the bores are drilled through the bone mass at the center of the hexagon pattern.
U.S. Pat. No. 5,439,684 is directed toward various swollen demineralized bone constructions such as sleeves, rectangular pledgets and wedges. The pledgets and wedges can be used as invertebrate support blocks. The bone can be machined into a desired shape for implantation such as sheet, disc, ring, cube, cylinder or sliced and wrapped into a tubular shape. However, all of these bone structures are limited to the size and shape of the original material.
Another patent of interest is U.S. Pat. No. 4,858,603 which shows a bone pin which is placed through an angular bore cut through two separate pieces of bone to hold the pieces together in a fixed secured relationship. The bone pin is made from a polymer which is absorbable in an animal body.
Until now, the only way that separate bone pieces could be joined together to arrive at a larger device has been to tongue and groove the respective pieces which creates shearing areas and limits the use to which such constructed device could be used. This is a significant problem where a device is placed under stress and shearing forces as for example where it is hammered into place between vertebrae or into other bone areas. The first disclosure of joining together separate pieces of allograft bone is believed to be set forth in various articles by F. Albee. This disclosure also show the machining of dowels, pins and screws from bone. F. Albee, Bone Graft Surgery in Disease, Injury and Deformity p. 22 (1940); and F. Albee, The Improved Albee Bone Mill, American Journal of Surgery p. 657 (March 1938).
Accordingly, there is a need for implantable shaped structures made entirely out of cortical bone tissue that are larger than naturally occurring bone structures and are made wholly out of cortical tissue without using external, non-cortical fasteners or adhesives.